Transforming growth factor-[beta]1 downregulates caveolin-1 expression and enhances sphingosine 1-phosphate signaling in cultured vascular endothelial cells

In vascular endothelial cells, specialized microdomains of plasma membrane termed caveolae modulate various receptor signal transduction pathways regulated by caveolin-1, a resident protein of caveolae. We examined whether transforming growth factor-[beta]1 (TGF-[beta]1), a multifunctional cytokine, alters expression levels of caveolin-1 and influences heterologous receptor signaling. Treatment of cultured bovine aortic endothelial cells (BAEC) with TGF-[beta]1 induces marked decreases in caveolin-1 expression in a time- and dose-dependent fashion at both levels of protein and mRNA. A pharmacological inhibitor of activin receptor-like kinase 5 (ALK-5) counteracts caveolin-1 downregulation by TGF-[beta]1, indicating the involvement of ALK-5 receptor subtype for TGF-[beta]1. Sphingosine 1-phosphate (SIP) is a serum-borne angiogenic lipid growth factor that exerts a wide variety of biological actions. S1P modulates G protein-coupled S1P receptors, activating downstream molecules kinases AMP-activated protein kinase (AMPK), and Akt as well as a small G protein Racl, ultimately to promote migration. Because SIP receptor signaling is associated with caveolae/caveolin-1, we examined whether pretreatment with TGF-[beta]1 enhances effects of SIP on BAEC. Whereas SIP alone evokes robust BAEC responses to S1P, pretreatment with TGF-[beta]1 leads to even higher magnitudes of SIP-elicited signaling responses and cell migration. Conversely, genetic knockdown of caveolin-1 using small interfering RNA mimics TGF-[beta]1-induced promotion of BAEC responses to SIP. Collectively, these data demonstrate that TGF-[beta]1 downregulates caveolin-1 of cultured endothelial cells, involving ALK-5 receptor subtype. Because downregulation of caveolin-1 by TGF-[beta]1 promotes subsequent heterologous receptor signaling by SIP, these results may also identify novel point of cross-talk between cytokines and sphingolipids within endothelial signal transduction machineries. cytokines; sphingolipids; G protein-coupled receptors; endothelium; signal transduction doi: 10.1152/ajpcell.00109.2009.